How to repeat a texture mirrored?
In order to repeat the texture, at the same time mirroring it, you should check 2 options in the Image Mapping scroll:


increase value of the Repeat slider to something more, than 1 (otherwise Mirror options will continue being disabled, like you've mentioned);
check Mirror X and / or Mirror Y checkboxes.




The texture will be repeating as the pattern, mirrored on selected axes.